Since the time of these early experiments, various other modes of detecting the existence of electric waves have been found out in addition to the spark-gap which he first employed, and the results of his observations, the earliest interest of which was simply that they afforded a confirmation of an abstruse mathematical theory, have been applied to the practical purposes of signalling over considerable distances (see Telegraphy, Wireless).
Theoretically for a given outside diameter of core the greatest speed of signalling through a cable is obtained when the diameter of the conductor is 606 (1/,/e) the diameter of the core, but this ratio makes the thickness of the guttapercha covering insufficient for mechanical strength.
In this the tongue of the relay is kept over to the spacing side by means of a current flowing in one direction, but on the depression of the signalling key the current is reversed, moving the relay tongue over to the marking side.
If the positive is called the signalling current, the line will be charged positively each time a signal is sent; but as soon as the signal is completed a negative charge is communicated FIG.
The actual speed or rate of signalling is given approximately by the formula, S = 120/ (KR), where S is the number of words per minute, R the total resistance of the conductor in ohms, and K the total capacity in farads.
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